Integrated circuits are being formed on smaller and thinner semiconductor die for a variety of reasons and applications. Relatively thin integrated circuits (ICs) or semiconductor die, also known as “ultra-thin” or “super-thin” ICs or die (also referred to as thin die below), are used in applications such as smart cards, smart labels, sensors and actuators. One example of a thin die application is for pressure sensors wherein a thin die containing a piezoresistive circuit is mounted on the top of a diaphragm in a pressure port to sense pressure.
When making and handling a thin die, care must be taken not to fracture or otherwise damage the die. A need therefore exists for improved methods and procedures to fabricate, separate, and transport thin dice for high volume applications where automated techniques are required to produce high throughput and acceptable yields.
It is already known to separate and handle integrated circuits on thin semiconductor wafers by mechanical grinding, chemical etching and dry etching with the assistance of adhesive or UV-reactive release tapes and carrier wafers. Some of the approaches taken in the electronics industry to separate thin wafers into die and to handle thin die include dicing by cutting and dicing by thinning. In dicing by cutting, a dicing tape is mounted on frames. The wafers are mounted to the dicing tape, backside down. Dicing is carried out by sawing, laser cutting and/or dry etching. After cutting, the die are separated on the dicing tape and sent to the assembly line on a wafer frame for pick and place. The thin dice are then ejected from the backside of the tape with the help of an ejector pin and picked by a vacuum tip. An example of this process flow is described in Muller et al., “Smart Card Assembly Requires Advanced Pre-Assembly Methods,” SEMICONDUCTOR INTERNATIONAL (July 2000) 191.
In dicing by thinning, trenches are etched or sawed on the topside of a device wafer. Laminating tapes are then placed on a carrier wafer for mounting the carrier wafer to the topside of the device wafer. The bottom side of the device wafer is then thinned until the topside trenches are opened from the bottom side. A second carrier wafer is then mounted to the bottom side of the device wafer by a high-temperature release tape. The first carrier wafer is removed and then the thin die can be removed by locally heating a vacuum-picking tool. An example of this process flow requiring multiple carrier wafers and tape transfers is described in C. Landesberger et al., “New Process Scheme for Wafer Thinning and Stress-Free Separation of Ultra Thin ICs,” published at MICROSYSTEMS TECHNOLOGIES, MESAGO, Dusseldorf, Germany (2001).
Alternatively, it has been known to saw or cut a carrier wafer into carrier chips, each of them carrying a thin die. In this case, the carrier chip is removed after die bonding by thermal release of the adhesive tape. An example of this process flow is described in Pinel et al., “Mechanical Lapping, Handling and Transfer of Ultra-Thin Wafers,” JOURNAL OF MICROMECHANICS AND MICROENGINEERING, Vol. 8, No. 4 (1998) 338.
Conventional procedures have been met with a varying degree of success. The combination of carrier transfers and tape transfers necessitate multiple steps with long cycle times and yield loss. Moreover, the use of heat release and other tapes may exhibit unacceptable residual adhesion. When used in combination with an ejector pin, the edges may not delaminate from the tape due to the lack of flexural rigidity of the thin die and due to the die's small size in the in-plane directions. The small size of the die may also limit the net suction force that could be exerted by the vacuum tip to overcome residual tape adhesion. Conventional dicing and wafer sawing methods often damage thin die, which cause device failure or sensor performance degradation. Conventional ejector pins may exert excessive stress that damages the thin die, also causing cracking and device failure. Carrier transfer or tape transfer may lead to die contamination on both sides of the die. Multiple transfers by wafer carriers typically lead to lower yield due to increased handling and contamination. In the case of a very thin die for sensor applications, organic adhesive may leave residue on the die surface, causing poor bonding with the surface being measured. It is, therefore, desirable to provide an improved device and method of fabricating, separating and handling very thin dice to overcome most, if not all, of the preceding problems.